Gusset Expansion Envelopes | C3 C4 C5 DL Peel Seal

When the contents are too thick for a standard envelope, a gusset expansion envelope provides the space without the need to move to a box. The expanding sides of a gusset envelope accommodate bulky documents, multi-page contracts, reports, catalogues, and legal bundles without straining the seams. Postal Packaging's range covers sizes from C6 through to C3, spanning 162x114mm up to 457x324mm, with peel and seal closure as standard on most lines. The gusset typically expands to 25mm, enough for a substantial document pack or several brochures. Available in white and manila finishes to suit both formal correspondence and practical dispatch.

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The Practical Solution for Thick Documents

A standard envelope is designed for a single sheet or a small number of pages. When contents exceed that, the envelope fails: the flap will not seal, the seams split, or the document folds under pressure. A gusset envelope solves this directly. The expanding base and sides create depth that a flat envelope cannot, and the peel and seal strip closes cleanly regardless of how much is inside.

When to Use a Gusset Envelope

Multi-page legal or financial documents that cannot be folded
Reports and proposals sent flat in C4 or C3 format
Catalogues, brochures, and promotional packs
Medical records or case files requiring a secure, flat envelope
Multiple documents dispatched together as a set

Size Guide

C6 (162x114mm): suitable for A6 documents folded twice, or small multi-page packs
DL (220x110mm): standard long document format with added depth for thick letters
C5 (229x162mm): A5 documents flat, or A4 folded once with several sheets
C4 (324x229mm): A4 flat, the most common size for reports and document packs
C3 (457x324mm): A3 flat, or large-format document bundles

Gusset Depth and Closure

Standard gusset depth is 25mm, which typically accommodates 20 to 40 sheets at 80gsm. Peel and seal closure is used on most lines for fast, secure sealing without moisture. String and washer gusset formats are also available for documents that require repeated access.

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Aimée Thompson

We ordered some branded envelopes for a client event and they were a perfect addition. Jess and the team were extremely accommodating and attentive throughout the ordering process, pushing our order through to meet the tight deadline. I really recommend this company for branded materials. Thank you team!

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Charlotte Goodman-Brooks

Great Customer Service

Unfortunately my order arrived damaged but after emailing the issue over to the company I received a quick response from Jess who was able to fix the problem in minutes, thanks so much jess, fantastic customer service, will definitely be ordering from this company again.

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Great Products, Quick Delivery

DPD were having issues delivering to my address and things were getting frustrating, but the company really stepped up. They sorted the problems with DPD quickly, the replacement arrived super fast, and they refunded my previous order without any hassle. Overall, I’m really impressed – amazing customer service, and I’ll definitely be ordering again!

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Grace

Proactive and Helpful

The courier let us down, but Jess and Kelly were hugely helpful and did everything they could to resolve it. They even arranged to specially stock products I order regularly to make future orders smoother. Great quality, good value and excellent customer service, with the phone answered on the first ring!

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Rebecca R

Really Easy Process

Really easy process to order some custom printed envelopes, James was so helpful and answered all my questions. The envelopes arrived on time and are really great quality, wouldn't hesitate to use this company again!

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Bernie Webber

Great Company To Buy From

Great company to buy from. Very helpful and communicative.

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Paul Craig

Prompt and Efficient Service

Very helpful staff directing me to exactly the right item needed.

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Richard

Fantastic Customer Service

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